eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2018-08-29
13:1
13:23
10.4230/LIPIcs.CSL.2018.13
article
Graphical Conjunctive Queries
Bonchi, Filippo
1
Seeber, Jens
2
Sobocinski, Pawel
3
University of Pisa, Italy
IMT School for Advanced Studies Lucca, Italy
University of Southampton, UK
The Calculus of Conjunctive Queries (CCQ) has foundational status in database theory. A celebrated theorem of Chandra and Merlin states that CCQ query inclusion is decidable. Its proof transforms logical formulas to graphs: each query has a natural model - a kind of graph - and query inclusion reduces to the existence of a graph homomorphism between natural models.
We introduce the diagrammatic language Graphical Conjunctive Queries (GCQ) and show that it has the same expressivity as CCQ. GCQ terms are string diagrams, and their algebraic structure allows us to derive a sound and complete axiomatisation of query inclusion, which turns out to be exactly Carboni and Walters' notion of cartesian bicategory of relations. Our completeness proof exploits the combinatorial nature of string diagrams as (certain cospans of) hypergraphs: Chandra and Merlin's insights inspire a theorem that relates such cospans with spans. Completeness and decidability of the (in)equational theory of GCQ follow as a corollary. Categorically speaking, our contribution is a model-theoretic completeness theorem of free cartesian bicategories (on a relational signature) for the category of sets and relations.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol119-csl2018/LIPIcs.CSL.2018.13/LIPIcs.CSL.2018.13.pdf
conjunctive query inclusion
string diagrams
cartesian bicategories